Question

A. Using given data, calculate the change in Gibbs free energy for each of the following reactions. In each case indicate whether the reaction is spontaneous at 298K under standard conditions.

2H2O2(l)→2H2O(l)+O2(g)
Gibbs free energy for H2O2(l) is -120.4kJ/mol
Gibbs free energy for H2O(l) is -237.13kJ/mol

B. A certain reaction has ΔH∘ = + 35.4 kJ and ΔS∘ = 85.0 J/K . Calculate ΔG∘ for the reaction at 298 K. Is the reaction spontaneous at 298K under standard conditions?

Answer 1

A)   2H2O2(l)→2H2O(l)+O2(g)

ΔG^orxn = ΔG_f^o(products) - ΔG_f^o( reactants)

= 2ΔG_f^o [H2O(g)]} + ΔG_f^o (O2) - 2ΔG_f^o [H2O2(l)]

= 2x -237.13kJ/mol + 0 - [ 2 x  -120.4kJ/mol ]

= -233.46 kJ/mol

Therefore, ΔG^orxn = -233.46 kJ/mol

Since, ΔG^orxn is negative - reaction is spontaneous at 298K under standard conditions.

B)

∆G^o = ∆H^o - T∆S^o

= + 35.4 x 10³ J/mol - [298 K x 85.0 J/mol.K]

= +10070 J/mol

∆G^o = +10070 J/mol

Since  ΔG^o is positive - reaction is non-spontaneous at 298K under standard conditions.